Magnetic cores used in motors, transformers, and the like are required to have high magnetic flux density and low iron loss. Conventionally, electrical steel sheets have been stacked in such magnetic cores, yet in recent years, dust cores have attracted attention as magnetic core material for motors.
The most notable characteristic of a dust core is that a 3D magnetic circuit can be formed. Since electrical steel sheets are stacked to form a magnetic core, the degree of freedom for the shape is limited. A dust core, on the other hand, is formed by pressing soft magnetic particles coated with insulation coating. Therefore, all that is needed is a die in order to obtain a greater degree of freedom for the shape than with electrical steel sheets.
Press forming is also a shorter process than stacking steel sheets and is less expensive. Combined with the low cost of the base powder, dust cores achieve excellent cost performance. Furthermore, since the surfaces of the electrical steel sheets are insulated, the magnetic properties of the electrical steel sheet in the direction parallel to the steel sheet surface and the direction perpendicular to the surface differ, causing the magnetic cores consisting of stacked electrical steel sheets to have the defect of poor magnetic properties in the direction perpendicular to the surface. By contrast, in a dust core, each particle is coated with insulation coating, yielding uniform magnetic properties in every direction. A dust core is therefore appropriate for use in a 3D magnetic circuit.
Dust cores are thus indispensable material for designing 3D magnetic circuits, and due to their excellent cost performance, they have also been used in recent years from the perspectives of reducing the size of motors, reducing use of rare earth elements, reducing costs, and the like. Research and development of motors with 3D magnetic circuits has thus flourished.
When manufacturing high-performance magnetic components using such powder metallurgy techniques, there is a demand for components to have excellent iron loss properties after formation (low hysteresis loss and low eddy current loss). These iron loss properties, however, are affected by the strain remaining in the magnetic core material, impurities, grain size, and the like. In particular, among impurities, oxygen is an element that greatly affects iron loss. Since iron powder has a greater oxygen content than steel sheets, it is known that the oxygen content should be reduced insofar as possible.
Against this background, JP 2010-209469 A (PTL 1), JP 4880462 B2 (PTL 2), and JP 2005-213621 A (PTL 3) disclose techniques for reducing the iron loss of magnetic core material after formation by reducing the oxygen content in iron powder to less than 0.05 wt %.